Pre treatment of multi-phase materials using high field strength electromagnetic waves

ABSTRACT

The material ( 200 ) having a first phase of material and a second phase of material. The method comprises heating the material electromagnetically, preferably with microwaves ( 202 ), to produce a power density of at least 10 9  Wm −3  in a continuous process in which the material ( 200 ) moves into and through an electromagnetic, preferably microwave, treatment area ( 212 ). The material ( 200 ) experiences exposure to microwaves ( 202 ), in the treatment area ( 212 ) for a time of the order of ½ second or less before the material ( 200 ) is passed out of the treatment area ( 212 ) for subsequent operation.

This invention relates to the production of high electric field strengthelectromagnetic radiation, typically but not necessarily microwaveradiation and typically but not necessarily for the weakening ofmulti-phase materials using microwaves.

The invention arises from a consideration of how to process mined oresand it is convenient to illustrate it in that context. It will berealised that the invention has wider applications.

It is known to process, e.g. by milling, ores to extract a wantedmineral from unwanted surrounding rocks or minerals, comminution of oresis a well-established industry. Milling or grinding ores is very energyintensive. It has been estimated that one and a half percent of allenergy used in the United States is used in the comminution of ores andminerals. It is very big business.

There are many suggestions as to how to pre-treat materials before theyare processed by a milling/grinding machine. Some involve chemicaltreatment, some involve heat treatment, and there are proposals, but asyet unsuccessfully implemented, to pre-treat with microwaves. There isalso a proposal to use electric discharges. The prior art, bothimplemented and speculative, points in many, often contradictory,directions.

Some literature in the field includes:—U.S. Pat. No. 5,824,1533, PCTPatent Application WO 92/18249, British Patent Application No. GB 2 120579, and the papers “The Influence of Minerology on Microwave AssistedGrinding”, S. W. Kingdom, W. Vorster and N. A. Rowson, MineralEngineering Vol. 13, No. 2, Elsevier Science Limited, 0892-6875 (99)00010-8; “Effects of Microwave Radiation upon the Mineralogy andMagnetic Processing of a Massive Norwegian Ilmenite Ore” by S. W.Kingman, G. M. Corfield and N. A. Rowson, Magnetic and ElectricalSeparation, Vol. 9. published by Overseas Publishers Association N.V.;“The Effects of Microwave Radiation on the Processing of Palabora CopperOre” by S. W. Kingman, W. Vorster and N. A. Rowson, published by TheJournal of the South African Institute of Mining and Metallurgy,May/June 2000; “Microwave Treatment of Minerals—A Review”, by S. W.Kingman and N. A. Rowson, published by Minerals Engineering, Vol 11,Elsevier Science Limited, 0892-6875 (98) 00094-6; “The Effect ofMicrowave Radiation on the Processing of Neves Corvo Copper Ore” by W.Vorster, N. A. Roswon and S. W. Kingman, International Journal ofMineral Processing 63 (2001) 29-44 published by Elsevier Science B.V.;“Short-Pulse Microwave Treatment of Disseminated Sulfide Ores” by J. B.Salsman, R. L. Williamson, W. K. Tolley and D. A. Rice, MineralsEngineering, Vol. 9, No. 1, 1996 published by Elsevier Science Limited0892-6875 (95) 00130-1; “The Effect of Microwave Radiation on theMagnetic Properties of Minerals” by S. W. Kingman and N. A. Rowson,Journal of Microwave Power and Electromagnetic Energy Vol 35, No. 3,2000; “Applications of Microwave Radiation to Enhance Performance ofMineral Separation Processes” by S. W. Kingman, N. A. Rowson and S.Blackburn, IMN 1997 ISBN-1870706388.

Many of these discuss having conventional multi-mode microwave producingmachines applying microwaves for quite long periods (10 seconds or muchlonger) to batches of minerals, and then processing them by crushingand/or grinding.

It is reported in some of the above papers that the energy expended inmicrowaving minerals can be far more than the energy saved in thecomminution process.

Some of the proposals have few experimental facts and are largelytheory, and some have experimented not on a real ore but a groundmixture of two minerals to assess their thermal performance, but not thestress at the boundary between minerals. Some predict temperature risesthat will melt or chemically alter the minerals concerned, making itdifficult or impossible to separate the mineral economically and aretherefore unappealing.

The above means that in practice a designer of a mineral processingplant does not consider microwave pre-treatment as being at allfeasible/desirable. It is not currently seen as being a way to reduceoverall costs. There is a prejudice in the art away from usingmicrowaves. It is not known that there is even a single production-scalefacility that uses pre-treatment by microwaves as a conditioning step inthe treatment of ores prior to comminution.

The UK Patent Office has conducted a search and has found the followingdocuments:—

GB 2205559 (Wollongong Uniadvice Ltd.) discloses a method of drying andheating ores where heat is conducted using a carbon phase material.

EP 0041841 (Cato Research Corporation) discloses a process usingmicrowave energy to chemically change a compound to aid extraction fromthe ore.

WO 97/34019 (EMR Microwave Technology Corporation) discloses a methodfor bringing about a metallurgical effect in a metal-containing ore.

WO 92/18249 (The Broken Hill Proprietory Company Ltd.) discloses aprocess for recovery of a valuable species in an ore which has a processtime of up to 1 hour exposing the ore to pulses of microwave energy of 1to 30 seconds duration with intervals of 10 seconds to 2 minutes betweenpulses.

U.S. Pat. No. 5,003,144 (Lindroth) discloses apparatus involving the useof microwave radiation for pre-weakening a mineral. Extended use ofmicrowave radiation leads to substantial heating of the mineral, whichcan in turn lead to chemical changes occurring in the mineral, anddegradation of the desired mineral.

According to a first aspect of the invention we provide a method ofmicrowave pre treatment of a multi-phase material prior to a subsequentoperation on the material, the material having a first phase of materialand a second phase of material, the method comprising heating thematerial electromagnetically at a power density of at least 10⁹ Wm⁻³ ina continuous process in which the material moves into and through anelectromagnetic treatment area and experiences exposure toelectromagnetic energy in the treatment area for a time of the order of½ second or less, and passing the material out of the treatment area forsaid subsequent operation.

An important application of the invention is in mineral processing toweaken the bond between a first phase of material and a second phase ofmaterial in a multi-phase composite material. For example, ores orminerals that are desired to be extracted are found in a different phaseof rock.

By using microwaves to heat two phases in a material (e.g. rock)differentially it is possible to have differential expansion over thetwo phases, and to cause cracks or weakening of their interface. Thiscan facilitate the extraction of the mineral from the rock. There ispreferably still post-microwave treatment of the ore to extract thedesired material, for example mechanical pre-treatment of the ore orrock to separate the first and second phase materials.

We have also discovered a very interesting, commercially useful, effect.It is necessary to heat multi-phase materials (or other materials) withmicrowaves for far less time than is previously been thought desirable.We may expose the material to high intensity microwaves first forsomething of the order of a second or less, but in all probability ofthe order of ½ second or less, or the order of a quarter of second less,or the order of 0.1 of a second or less, or of the order of 0.01 secondor less, or of the order of 0.001 second or less, or possibly even theorder of 0.0001 second or less. Depending upon the choice of first andsecond phase materials, about 1 ms of exposure of a material in amicrowave application zone (or less) may be desirable. For otherapplications exposure in a microwave zone to microwaves for a time ofthe order of 0.1, or 0.2, of a second may be the best weakening effectfor power expenditure with a power density appropriately high. Typicalpower density that we would have in mind might be about 10¹² watts percubic metre or above, or better still 10¹⁵ or 10¹⁶ Wm⁻³ or above.

It will be appreciated that material may be in a treatment zone/passthrough it for a period of time that is longer, or much longer, thanthat for which the material is actually exposed to electromagneticradiation.

We have also appreciated that it is possible to pass material through amicrowave cavity in a continuous stream, for a continuous treatmentprocess. The microwave cavity has high electric field which in turnproduces high power densities (e.g. 10¹⁵ Wm⁻³ or 10¹⁶ Wm⁻³ or more) andmaterial can be made to move through high field strength electromagneticwaves, residing in the high intensity region for only a short time. Thishas the double benefit of increasing the throughput of materials throughthe treatment machine, and using the knowledge that we do not need toapply microwaves to materials for very long to achieve the desiredeffect. The two advantages have synergistic effect.

In some embodiments the method comprises creating a standing wave ofmicrowaves in a cavity and ensuring that the composite material isdisposed in the cavity at a position on or about a maximum intensity ofthe standing wave.

The method may have a guide means which guides the composite material tothe position of a maxima of the standing wave.

According to another aspect of the invention we provide a method ofweakening the bond between a first phase of material and a second phaseof material in a multi-phase composite material comprising applying ahigh powered density of microwave, or high electric field strengthmicrowaves, to the composite material for an exposure time that is ofthe order of a ½ or ¼ of a second or less.

By order of ½ or ¼ of a second or less in the above definition we meanin some embodiments to exclude 1 second, and in others to still includeabout 1 second.

According to another aspect of the invention we provide apparatus formicrowave treatment of material comprising:

-   -   a microwave treatment zone;    -   a microwave emitter disposed at said microwave treatment zone;    -   a material transporter adapted to transport material through the        microwave treatment zone; the arrangement being such that:—    -   the microwave emitter is adapted to emit microwaves at a power        density of at least 10⁹ Wm⁻³;        and the material transporter is adapted to transport said        material through the microwave treatment zone fast enough so        that said material experiences applied microwaves in said zone        for a time of the order of ½ second or less.

According to another aspect of the invention we provide a method ofmicrowave processing material comprising applying a high power densitymicrowave, or high electric field strength microwave, to the materialfor an exposure time that is of the order of ½ or ¼ of a second or less.

According to another aspect of the invention we provide apparatus forprocessing a material comprising a microwave cavity adapted to applyhigh power density microwaves to the material for an exposure time thatis of the order of ½ or ¼ of a second or less.

Preferably the exposure time is achieved by passing the material througha microwave cavity at a speed so as to achieve the desired exposuretime.

According to another aspect of the invention we provide apparatus forweakening the bond strength between a first phase of material and asecond phase of material in a multi-phase composite material comprisinga microwave cavity adapted to apply high power density microwaves to thecomposite material for an exposure time that is of the order of ½ or ¼second or less.

We may expose the ore to microwaves or other radiation for 1 second orso, or longer, after all, and protection for that is also sought.

According to another aspect of the invention we provide a method ofcontinuous processing of ore or rocks comprising applying high electricfield strength microwaves to create high power densities, on acontinuous basis to ore or rocks passing through a microwave cavity orzone to weaken the ore or rocks, and subsequently passing the continuousflow of ore or rocks to a mechanical treatment machine and mechanicallybreaking up the ore or rocks.

The microwaves may be pulsed, and applying them on a continuous basis isnot meant to exclude repeated pulses of microwaves.

A reduction in overall energy consumption—quite a serious reduction—maybe available if we pre-treat the ore or rocks with microwaves so as toweaken them and then break them up in a mechanical comminution process.

Moreover, a continuous process has a higher throughput, and can copewith higher volumes than batch processes. This makes the process evenmore economically attractive.

It is particularly elegant that once we have a high enough electricfield strength we can then flow material (whether that be for weakeningthe bond between different phases, or other purposes) through themicrowave field in a continuous manner at a rate that is fast enough toexpose the material to the high intensity microwave for only a shorttime, (e.g. ½ or ¼ second or less, perhaps of the order of 1 ms), andthe fact that the material is exposed for a short time reduces the costper unit of material, the fact that there is a continuous processimproves the throughput, the fact that the materials have to flow quitefast through the microwave cavity/zone improves the throughput, and allof these things reduce the cost of the processing per unit of materialprocess.

The electric field strength of the microwaves and the time of exposurenecessary to cause weakening/differential heating are related; thehigher the field strength the shorter need be the exposure time.

According to another aspect of the invention we provide apparatus forcontinuous processing of ore or rocks comprising means for applying highelectric field strength microwaves to create high power densities, on acontinuous basis to ore or rocks and feed means adapted to passsubsequently the continuous flow of ore or rocks to a mechanicaltreatment machine adapted mechanically to break up the ore or rocks.

We have appreciated that a higher temperature gradient is needed toseparate ores and minerals from the surrounded unwanted material.

According to further aspects of the invention we provide a method ofweakening the interface between a first phase of material and a secondphase of material comprising creating a temperature gradient at aninterface between the first and second phases of at least 100° C.,possibly by using a standing wave of microwaves to heat the first andsecond phases differentially.

According to another aspect of the invention we provide apparatus forweakening the interface between, or separating, a first phase ofmaterial from a second phase material, the apparatus being capable ofcreating a temperature gradient at an interface between the first andsecond phases of at least 100° C., possibly by creating a standing waveof microwaves to heat the first and second phases differentially.

A single mode cavity may be provided to produce a standing wave.

According to another aspect of the invention we provide a method ofrapidly heating a material comprising creating a standing wave ofmicrowaves and a region of maximum electric field strength, and havingmaterial disposed in said region of maximum electric field strength.

We have realised that standard multi-mode microwave cavities, similar tothose found in conventional kitchen microwave ovens, have manyadvantages, are very commonly available and are the equipment of choicefor very many areas, but that they do not achieve maximum electric fieldstrength. Multi-mode cavities do not have a single standing wave createdin them—they deliberately “smear” their energy out uniformly across thecavity (or more or less uniformly) so as to achieve any effect evenly—ormore evenly—throughout the volume of the cavity. This has been the driveof multi-mode cavity designers. However, we have appreciated that therecan be times when processing a material when very high electric fieldstrengths are required and that the best way to obtain these, in theabsence of sufficiently powerful multi-mode cavity machines at areasonable cost, is to use a microwave cavity which can sustain, anddoes sustain, a single standing wave. This single standing wave then hasmaximum and minimum electric field regions, which coincide with maximumand minimum power density (there is a relationship between power densityand electric field strength and electric field strength varies with apower greater than 1 in comparison to power density—generally a squaredpower relationship). We have then appreciated that in order to apply themaximum electric field strength, produced by a typical microwavegenerator (or any particular specific microwave generator) it isdesirable to align the position of the material to be processed with theposition of the maxima in the standing wave. This can typically beachieved by controlling the position of the material relative to thecavity, but alternatively it is possible theoretically to move theposition of the maxima to suit the position of the material within thecavity, by appropriately tuning the standing wave. Preferably a singlemode microwave cavity is used. A single mode microwave cavity enables usto provide a good standing wave.

According to another aspect of the invention we provide a method ofweakening the bond between a first phase of material and a second phaseof material in a multi-phase composite material, the method comprisinginducing a high thermal gradient at an interface between the first andsecond phases by applying microwaves to create a power density of atleast 10⁹ watts per cubic metre, and creating a standing wave having anarea of high electric field strength and positioning the material at orabout the area of high electric field strength.

According to another aspect of the invention we provide a method ofmicrowave pre-treatment of a multi-phase material prior to a subsequentoperation on the material to extract one material from the other(s), themethod comprising providing a continuous feed of the multi-phasematerial through a region in which microwave radiation is present at aspeed to allow a throughput of multi-phase material of at least 500tonnes per hour, the microwaves creating a power density of at least10⁹, 10¹⁰, 10¹², 10¹³, or 10¹⁴ Wm⁻³, the material being present in themicrowave radiation region for a time during which time it experiences aplurality of pulses of microwave energy such as to expose the materialto microwaves for a summed duration exposure time of the order of a fewms, or 1 ms, or less, and wherein the overall bulk temperature of themulti-phase material does not rise by more than about 40° C., andwherein a thermal stress is created between phase boundaries which isstrong enough to break bonds between the different phases, and whereinthere are no significant changes to the chemical properties of the phaseof materials to be extracted.

The microwaves may be applied in pulses of a duration of the order of afew μs, or tens or hundreds of μs, or less.

Embodiments of the invention will now be described by way of exampleonly, with reference to the accompanying drawings, of which:—

FIG. 1 a schematically illustrates a two-phase rock having crystals of afirst material embedded in a second material;

FIG. 1 b shows schematically the rock of FIG. 1 a after treatment bymicrowaves according to the present invention;

FIG. 2A shows schematically a mineral extraction plant and process inaccordance with the present invention;

FIG. 3A shows schematically a microwave pre-treatment unit for use inthe apparatus of FIG. 2;

FIG. 3B shows how electric field varies across the material inlet of theunit of FIG. 3A;

FIGS. A and 4B show variations of the unit of FIG. 3A;

FIG. 5 schematically illustrates a model of a calcite and pyrite oresample;

FIG. 6 illustrates dielectric loss factor versus temperature;

FIG. 7 illustrates variation of microwave power density versustemperature;

FIG. 8 illustrates the direction of simulated loading in a uniaxialcompression test;

FIG. 9 illustrates temperature distributions of a 2.45 GHz, 2.6 kWmicrowave cavity;

FIG. 10 illustrates the effect of varying heating times;

FIG. 11 illustrates the effect of microwave heating time on unconfinedcompressive strength;

FIG. 12 illustrates shear plain development during unconfinedcompressive tests;

FIG. 13 illustrates temperature distribution for a microwave cavity witha power density of 10¹¹ W per cubic metre;

FIG. 14 illustrates stress versus strain curves for different heatingtimes;

FIG. 15 illustrates unconfined compressive strength versus heating timefor a power density of 10¹¹ W per cubic metre;

FIG. 16 illustrates shear plain development during unconfinedcompressive tests for power density of 10¹¹ W per cubic metre;

FIG. 17 illustrates point of load index versus heating time for a powerdensity of 10¹¹ W per cubic metre;

FIG. 18 illustrates point of load index versus heating time fordifferent power densities;

FIG. 19 illustrates t10 versus ECS;

FIGS. 20A to 20C show further variations of the unit of FIG. 3A;

Table 1 shows specific heat capacity as a function of temperature;

Table 2 shows thermal conductivity as a function of temperature;

Table 3 shows thermal expansion co-efficient as a function oftemperature;

Table 4 shows mechanical properties of different minerals;

Table 5 shows the effect of different heating times on temperature andcompressive strength of material;

Table 6 shows similar factors to Table 5, but for a higher powerdensity;

Table 7 illustrates breakage parameters for a multimode cavity powerdensity between 3×10⁹ W per cubic metre and 9×10⁹ W per cubic metre;

Table 8 shows breakage parameters for a single mode microwave cavitywith a higher power density; and

Table 9 is a list of references referred to.

FIG. 1 a shows rock material 10 comprising crystals 12 of a firstmaterial embedded in a matrix 14 of a second material. An example of thefirst and second materials might be metal oxides (e.g. magnetite,ilmenite or haematite), or metal sulphides (e.g. copper, iron, nickel,zinc, or lead) as the first material, and possibly silicates, feldspars,or calcite as the second materials. It will be appreciated that theseexamples are non-binding and are illustrative only. There could bethird, or fourth, or subsequent, materials 16 also present in the rockmaterial 10. Thus, the rock material 10 comprises multiple phases ofmaterial having grain boundaries 18 between them.

FIG. 1 b shows the rock material 10 after it has been treated withmicrowaves in accordance with the present invention. The crystals, orregions, of the first material 12 now have a weaker bond to the material14, because the grain boundaries have been weakened due to the presenceof cracks/dislocations/areas of stress and strain. These are referenced20. In addition, there are also cracks 22 within the first materialregions 12 and cracks 24 in the second material 14.

The precise nature of grain boundaries between two mineral phases inrock is not well understood, but it is suggested to be an area ofdisorder between two ordered species. If this were the case, then itwould be sensible to assume that grain boundaries are an area ofweakness. However, products of comminution suggest that grain boundariesare an area of strength (transgranular fracture being common in mineralprocessing operations) and can adversely influence liberation of onespecies from another. Thus, whilst theory might say that grainboundaries should be an area of weakness, practice in traditionalcomminution suggest that grain boundaries are particularly strong.However, it has been postulated that if microwave energy can inducemicro-cracking around grain boundaries then reductions in requiredcomminution energy and enhanced liberation of a valuable mineral wouldoccur.

The reason why it is expected that cracks would occur at the grainboundary is due to the differential heating of the two material phases.They are expected to absorb energy from microwave differentially, and tochange temperature at different rates, inducing thermal stresses.However, to date this has not really happened economically.

With the present invention, it has been realised that the reason whythis has not happened is due to the temperature gradient not being largeenough between the different phases of material. We have realised thatto obtain a greater temperature gradient we should use a higher electricfield strength/power density. The sort of power density we have in mindis perhaps of the order of 10¹⁶ Wm⁻³, 10¹⁵ Wm⁻³, or 10¹⁴ Wm⁻³, or 10¹⁴Wm⁻³ (for example) for some applications. Depending upon the cavitydesign and dielectric of the material we may be generating electricfields of the order of 10⁵ Vm⁻¹ to 10⁷ Vm⁻¹, perhaps in the range of0.05×10⁶ Vm⁻¹. These figures are of course exemplary only and arenon-binding and are not intended to be restrictive.

Numerical modelling has been undertaken using the geomechanical 2-Dfinite difference modelling software application, FLAC V3.3 (Itasca1995). The model domain consisted an area representing a 15 mm wide by30 mm high section, which was subdivided into individual square zones of0.04 mm sides. The positions of the pyrite particles within the modeldomain were randomly generated to provide a relatively disseminated orebody, see FIG. 5. This type of dissemination has previously been shownto be responsive to microwave heating. It is appreciated that the‘mineralogy’ or texture used for the modelling may be a simplifiedversion of reality. However, the purpose of the investigation is todetermine the influence of power density on the degree of strengthreduction, not mineralogy. Therefore, as long as the mineralogy ortexture is the same for both tests the data can be truly comparative.What is important, however, is that the simulated ore contains speciesthat are both responsive and non responsive to microwave heating.

The finite difference modelling comprised of the 5 main stages givenbelow and more fully described later:

-   1. Microwave heating of the two different mineral phases-   2. Transient heat conduction during heating process between minerals-   3. Determination of peak thermally induced stresses and strains-   4. Modelling of thermal damage associated with material failure and    strain softening-   5. Simulation of uniaxial compressive strength tests to evaluate the    reduction of unconfined compressive strength due to microwave    heating.    Stage 1: Microwave Heating

The amount of thermal energy deposited into a material due to microwaveheating (power absorption density) is dependent on the internal electricfield strength, the frequency of the microwave radiation, and on thedielectric properties of the material.

The power absorption density per unit volume of the mineral can beapproximated from Equation 1.P_(d)=2.π.f.ε_(o).ε″_(r).E_(o) ²  (1)Where

-   P_(d) is the power density (watts/m³)-   f is the frequency of the microwave radiation (Hertz)-   ε_(o) is the permitivity of free space (8.854×10⁻¹² F/m)-   ε″_(r) is the dielectric loss factor of the mineral-   E_(o) is the magnitude of the electric field portion of the    microwave radiation (volts/m)

Because the microwave absorption factor for calcite is substantiallylower that that for pyrite no microwave heating of the calcite matrixwas assumed during the modelling with selective heating of the pyriteparticles only. The early work of Chen. (1984) and Harrison (1997) showsthis assumption to be realistic.

The dielectric loss factor, e″_(r), for pyrite has been found to bedependant on temperature (Salsman 1995). In determining the powerdensity for the pyrite the relationship between ε″, and temperature asshown in FIG. 6 was utilised (Salsman 1995).

For an initial series of models the power densities at varioustemperatures was obtained for the heating of pyrite within a 2.6 kW,2.45 GHz multimode microwave cavity. The calculated power density variedbetween 3×10⁹ watts/m³ at 300° K. and 9×10⁹ watts/m³ for temperaturesgreater than 600° K. (FIG. 7) (Kingman 1998). The initial temperature ofthe ore body sample was taken to be 300° K.

Stage 2 Modelling of Transient Heat Conduction During Microwave Heating

The transient conduction of the microwave thermal energy during heatingwas modelled using an explicit finite difference method written as analgorithm.

The basic concept in the thermal conduction modelling was that a thermalenergy flux may occur between a zone and its four immediately adjacentzones. The direction, i.e. into or out of the zone, and the magnitude ofthe thermal energy flux was dependent on the temperature gradient thatexisted between the zones and the conductivity of the zone. The boundaryconditions were such that no thermal energy was lost from the materiali.e. the material was assumed to be fully insulated.

The basic law that was used to determine the thermal energy flow betweenthe zones was Fourier's law, which has been given as Equation 2:—q=K.T_(diff)  (2)Where

-   -   q is the heat flux vector in joules/sec/m    -   K is the thermal conductivity tensor in w/m.° C.    -   T_((diff)) is the temperature difference (° C.)

Thus the change in stored energy per time increment, Δt, is given byEquation 3Δβ=Δt.p  (3)Δβ=Δt.q Where Δβ is the change in stored energy (Joules)

Expressing this in an explicit finite difference form for a square zonei,j with side length l:

Δβ_((i,j)) =Δt.K _((i,j)) l.[T _((i,j)) −T _((i,j−1)))+(T _((i,j)) −T_((i,j+1)))+(T_((i,j)) −T _((i+1,j)))+(T _((i,j)) −T _((i−1,j)))]  (4)

Where

-   -   K_((i,j)) is the thermal conductivity of zone i,j    -   Δt is the time increment in seconds    -   l is the length of the sides of the zones    -   T_((i,j)) is the temperature of zone i,j

The relationship between thermal energy in joules and temperature in °K. for a given time increment, Δt, is given by Equation 5:—$\begin{matrix}{{\Delta\quad T_{({i,j})}} = \frac{{\Delta\beta}_{({i,j})}}{m_{({i,j})} \cdot C_{({i,j})}}} & (5)\end{matrix}$where

-   -   ΔT(i,j)=temperature change in zone i,j (° K.)    -   m(i,j)=mass of zone i,j (Kg)    -   C(i,j)=specific heat of zone i,j (joules/Kg.K)

Thus at the end of each time increment the new temperatures of each zonedue to thermal conduction and microwave heating are determined usingEquation 6T _((i,j))(1)=300° K. T _((i,j))(n+1)=T _((i,j))(n)+ΔT _((i,j)) +Pd_((i,j))/(C _((i,j)) .Δt)  (6)Where

-   -   T_((i,j))(n) is the temperature of zone i,j at time increment n    -   Pd_((i,j)) is the power density of zone i,j

The microwave heating and thermal conduction for a specified heatingtime, ht, was simulated by recursively iterating Equations 4, 5 and 6until Equation 7 was satisfied.ht=n.Δt  (7)Where:

-   -   n time increment number    -   Δt is the time increment in seconds    -   ht is the heating time in seconds

The time increment, Δt, was restricted to 2.5×10⁻⁴ seconds to ensurenumerical stability, which itself corresponds to a measure of thecharacteristic time needed for the thermal diffusion front to propagatethrough a zone.

The thermal conductivity and specific heat properties of calcite andpyrite vary with temperature (Harrison 1997) and have been included asreference in Tables 1 and 2.

Thermal/mechanical Coupling

Stage 3 Thermally Generated Strains and Stresses

At the end of the heating interval the thermally induced strains withina zone, assuming perfect restrainment by the surrounding zones andisotropic expansion is given by Equation 8.ε_((i,j))=−α_((i,j)).(Tn _((i,j)) −T 1 _((i,j)))  (8)Where

-   -   ε_((i,j)) is the strain in zone i,j    -   α(i,j) is the thermal expansion coefficient (1/° K.) of zone i,j    -   Tn_((i,j)) is the final temperature of zone i,j    -   T1 _((i,j)) is the initial temperature of zone i,j

The thermal expansion coefficient for pyrite and calcite has also beenfound to be temperature dependant (Harrison 1997). Table 3 outlines thethermal expansion coefficient at various temperatures for calcite andpyrite as assumed and implemented within the modelling.

The calculated thermally induced stress within a zone can then bedetermined using Hoek's law for isotropic elastic behaviour (Equation9). $\begin{matrix}{\sigma_{({i,j})} = \frac{ɛ_{({i,j})} \cdot E_{({i,j})}}{\left( {1 - {2 \cdot \upsilon_{({i,j})}}} \right)}} & (9)\end{matrix}$Where

-   -   σ(i,j)=isotropic thermally induced stress within zone i,j        assuming perfect restrainment    -   E_((i,j))=Young's Modulus of zone i,j    -   ν_((i,j))=Poisson's Ratio of Zone i,j        Redistribution of Thermally Induced Stresses

To obtain a state of static mechanical equilibrium throughout the domainof the material a redistribution of the thermally induced stresses andstrains was necessary. To obtain the equilibrium distribution the modelwas stepped in FLAC's default calculation mode for static mechanicalanalysis. This default mode performs an explicit time-marching finitedifference calculation utilising Newton's law of motion to relate nodalstrain rates, velocities and forces (Itasca 1995). The material wasassumed to behave as a linear isotropic elastic medium with mechanicalproperties determined by the Young's Modulus, Poisson's Ratio anddensity (Table 4).

Stage 4 Modelling of Thermal Damage Associated with Material Failure andStrain Softening

When static equilibrium was obtained, modelling of the brittle fracture,where the stresses exceeded the strength of the material, was undertakenby simulating the constitutive behaviour of the ore body as anelasto-plastic material with plastic strain softening. The strength ofthe material was approximated as a very strong brittle crystallinelimestone with an unconfined compressive strength of 125 MPa and a shearstrength related by a linear Mohr-Coulomb strength criterion (Equation10).τ=σ_(n). tan φ+c  (10)Where

-   -   τ is the shear strength    -   σ_(n) is the normal stress acting normal to the shear plane    -   φ is the friction angle of the material    -   c is the cohesive strength of the material

Upon failure the material was assumed to behave as a brittle linearstrain softening medium undergoing plastic deformation with a finalresidual strength being obtained after 1% strain (Table 4).

Stage 5 Simulations of the Unconfined Compressive Strength Tests on theThermally Damaged Samples

The effect of thermal heating on the unconfined compressive strength andfracture development within the modelled material was predicted by thesimulation of the uniaxial compressive strength test on the thermallydamaged models (FIG. 8).

The simulation was undertaken as a plane strain analysis with thematerial being considered as continuous in the out of plane direction.The simulation was undertaken by applying a constant velocity to thegrid points positioned at the top and base of the model domain whilstthe left and right boundaries where unstrained. This is analogous to adisplacement controlled uniaxial compressive strength test. To monitorthe load-deformation relationship within the samples during testing,history files were generated of the average stress conditions at the topand bottom boundaries. The models were run until approximately 0.2%axial strain of the sample whereupon the models predicted failurestrength and some strain softening details of the samples was obtained.

Results of the Numerical Modelling

Microwave Heating Times

To determine the effect of microwave heating on the strength of thecalcite and pyrite ore, numerically modelling was undertaken for anunheated sample and for samples with microwave heating times of 1second, 5 seconds, 15 seconds and 30 seconds. It was assumed that thesamples were treated in a multimode microwave cavity with a powerdensity that varied from 3×10⁹ watts/ml at 300° K. to 9×10⁹ watts/m³ fortemperatures greater than 600° K.

Temperature Distributions

The modelled temperature distributions for each of the four timeintervals is shown in FIG. 9. It can be seen from the Figure that thehighest temperatures and temperature gradients were generated where thepyrite particles were clustered. Table 5 summarises the temperaturedistributions within the modelled samples for each temperatureincrement. Due to the length of time required to heat the pyriteparticles within the 2.6 kW microwave cavity, conduction of thedeposited thermal energy from the pyrite into the surrounding calcitehost was predicted to occur. After 30 seconds of microwave heating timethe calcite host had been heated to greater than 600° K. This conductioncan be seen to reduce the temperature gradient generated within the oresample and thus reduce the thermally generated stresses within thesample.

Effect of Microwave Heating on the Unconfined Compressive Strength

The effect of the microwave treatment on the unconfined compressivestrength of the ore sample has been illustrated in FIG. 10 andsummarised in Table 5. FIG. 11 shows the unconfined compressive strengthof the ore material plotted against microwave heating time and indicatesthat the heating intervals of 1 and 5 seconds had little affect on theunconfined compressive strength of the material. A more noticeablereduction in strength was predicted with microwave heating times of 15and 30 seconds. This observation may be attributed to the fact that therate of heating was insufficient to induce localised thermal gradientsof a magnitude that would generate thermal stresses that exceed thestrength of the ore material. Thus the modelled reduction in strength ofthe ore body may be attributed to the differential expansion of thepyrite and calcite material, due to different thermal expansioncoefficients, generating stresses that exceed the strength of thesample.

Pattern of Shear Planes

Also of interest was the pattern of the simulated shear planes developedwithin the modelled samples after the unconfined compressive tests.These patterns have been shown as FIG. 12 for the samples with microwaveheating times of 1, 5, 15 and 30 seconds. The fracture patternsdeveloped within the microwave heated samples were similar to thefracture patterns displayed by the unheated sample i.e. consistingmainly of continuous shear planes inclined at approximately 250 to thedirection of loading.

Effect of Increasing the Microwave Power Density

Power Density and Heating Time Intervals

To assess the effect of increasing the microwave power density on thetemperature distribution, unconfined compressive strength and shearplane development within the ore samples a microwave power density of1×10¹¹ watts/ml was assumed for the pyrite material. This power densitywas approximately 10 to 15 times greater than the power densitygenerated by using the 2.6 kW 2.45 GHz microwave cavity, although stilleasily within the range that can be achieved by microwave heating ofpyrite in a single mode cavity (Salsman 1995). It is assumed that thispower density is achieved by a single mode cavity supplied withmicrowave energy at a power level of 15 kW at 2.45 GHz (at this powerthis level of power density is easily achievable). The calcite hostmaterial was considered to be unheated by the microwave energy. Due tothe higher power density much shorter heating times of 0.05, 0.25, 0.5and 1 second were considered.

Temperature Distributions

The modelled temperature distributions within the ore samples for eachof the four time intervals are shown as FIG. 13. The Figure illustratesthat significantly greater temperatures were generated within the pyriteparticles. The shorter heating times compared to the 2.6 kW microwavecavity reduced the degree of thermal conduction, thus reducing theamount of heating of the calcite matrix. This generated temperaturegradients of a significantly higher magnitude within the ore samples.The temperatures within the ore samples obtained by the modelling havebeen summarised in Table 6.

Effect of Microwave Heating on the Unconfined Compressive Strength

The effect of the microwave heating on the unconfined compressivestrength of the ore samples is illustrated in FIG. 14. Compared to thereduction in strength within the 2.6 kW cavity it can be seen from FIG.15 that that the higher power density generates a considerably largerreduction in strength, with the majority of the strength reductionoccurring very quickly (within 0.05 seconds of microwave heating). Theresults of the modelling have been summarised in Table 6.

Pattern of Shear Planes

The pattern of shear planes developed within the ore samples after thesimulated uniaxial compression test, for the 0.05, 0.25, 0.5 and 1second heating intervals are shown as FIG. 16. The Figure indicates,unlike the unheated and 2.6 kW cavity heated samples, that the shearplanes are irregular and concentrated along the grain boundaries betweenthe pyrite and calcite. This may be attributed to the high thermallyinduced stress that develop along these boundaries due to the rapidlocalised heating and expansion of the pyrite particles within therelatively unheated calcite matrix.

Discussion

The influence of microwave power density on a theoretical ore has beendemonstrated. The numerical simulation has shown very clearly that ifthe preferential dielectric material can be made to absorb the majorityof the applied energy significant reductions in compressive strength canbe achieved. To further illustrate this in the context of comminutionthe extremely well known relationships developed by (Broch and Franklin,1972 and Bieniawski, 1975) were used to calculate the point load index(Is(₅₀)) from the modelled UCS data. The equation used was:—I _(s)(50)=UCS/K  (11)Where

-   I_(s)(50)=Point load strength corrected to 50 mm core.-   K=24-   UCS=Uniaxial compressive strength

The results of this analysis are shown in FIGS. 17 and 18. FIG. 17 showsthe influence of microwave heating time versus point load index for thelower power density. It can clearly be seen that as microwave exposuretime is increased the point load index decreases significantly. This isalso true in FIG. 18, which shows microwave heating time versus pointload index for the ore exposed to the higher density. As for the UCStests in FIGS. 11 and 15 the reductions in point load index areparticularly significant at the higher power density with a reductionfrom 5.25 for non-treated to 1.25 after just 0.2 seconds.

Point load index is of particular interest to the mineral processingengineer because it allows rapid prediction of the relationships betweenEcs (Specific comminution Energy KWh/t) and t₁₀ (t₁₀ is the percentagepassing 1/10^(th) of the initial mean particle size) (Bearman et al1997). The t₁₀ can be interpreted as a fineness index with larger valuesof t₁₀ indicating a finer product. However, in practise the value of t₁₀can be used to reconstruct the size distribution of the broken ore. Thet₁₀ value is related to the specific comminution energy by the followingequation (Napier-Munn et al. 1996):—t ₁₀ =A[1−e ^((−b.ecs))]  (12)Where A and b are material specific breakage parameters. A is thetheoretical limiting factor of t₁₀ and b is the slope of the ECS versust_(10 plot). Determination of A and b for a specific material can leadto calculation of a specific size distribution for a specific energyinput.

It has previously been shown that point load index is intimately relatedto Mode 1 fracture toughness (Bearman 1999). Bearman showed thatK_(ic)=0.209I_(s(50))  (13)Where

-   K_(ic)=Mode 1 Fracture Toughness (MN/m^(3/2))

Mode 1 fracture toughness has also been shown to have highly significantcorrelation with the breakage parameters A and b (Bearman et al 1997).

It was shown that:b=2.2465×K _(IC) ^(−1.6986)  (14)A.b=126.96×K _(IC) ^(1.8463) (15)

Table 7 shows the calculation of the breakage parameters for thetheoretical ore exposed to the 2.6 kW microwave radiation for times of 010 and 30 seconds. Table 8 shows the calculation of breakage parametersfor the same ore treated at the higher power density. This data was usedin conjunction with Equation 11 to calculate the influence of ECS ont₁₀. Energy inputs of 0, 0.25, 1 and 2.5 kWh/t were used for thecalculation. For clarity data is only presented for the non-treated andthe most extreme treatment times i.e. 30 seconds and 0.02 seconds. FIG.19 shows the influence of power density on the ECS v t₁₀ graph. It canbe seen that as power density is increased the slope of the plotincreases significantly and the theoretical limiting value of t₁₀ isreached for a much lower energy input. Put simply this means thattheoretical ore treated at the lower power density produces a muchcoarser product for a set specific comminution energy input than thattreated at the higher power density. If it is assumed that the mass ofmaterial heated is 1 kg the sample energy input for each case is for 2.6kW treated sample heated for 30 seconds in the multimode cavity:—2.6×0.5/60×1000/1=125 kWh/tand for the 15 kW treated sample heated in the single mode cavity for0.2 seconds:—15×3.33×10⁻⁻³/60×1000/1=0.8325 kWh/t.

This clearly shows the influence of power density on the comminution ofores.

The purpose of this discussion has been to illustrate the influence ofpower density (or electric field strength) on the comminution ofminerals. It is appreciated that the texture used for the modellingstage is not exactly like a ‘real’ ore. However, the ore has behaved ina similar manner to real ores previously tested (Kingman et al. 2000).Also the values obtained for the breakage parameter A are similar tothose expected for a typical hard rock ore (Napier Munn 1996). It hasbeen shown that increasing the power density the significantly greaterstresses are created for much lower energy inputs. This has significantramifications for the development of microwave assisted comminutionflowsheets. It is concluded that the use of high power density cavitiesmakes the microwave treatment of minerals economic, especially whencoupled to the additional benefits of thermally assisted comminution.

The references discussed are in Table 9.

The above theoretical discussion, which we are the first to realise hassignificance, has been followed up with actual trials of short duration,high field strength, standing wave microwaves on rock samples and theydo indeed break along crystal boundaries. Cracks have been seen alonggrain boundaries—which is very encouraging.

What we have realised is that the previous treatment of minerals hasused standard multi-mode microwave cavities, similar to those found inconventional microwave ovens. Whilst a multi-mode cavity is mechanicallysimple, it suffers from poor efficiencies and relatively low electricfield strengths. We have concluded that high electric field strengthsare vital to high power absorption and vital to causingcracking/weakening at the grain boundaries. We have concluded that it isnot appropriate to “gently” heat the different phases because thatallows time for temperature gradients to be smoothed out. What we wantis for a large temperature gradient to be created quickly, so as toinduce greater strain/stresses at the grain boundaries. This is achievedbetter by having high power density microwave radiation.

One way of achieving this is by not having standard multi-mode cavities,but rather having single mode cavities. These particularly comprise ametallic enclosure into which a microwave signal of correctelectromagnetic field polarisation is introduced, and undergoes multiplereflections. The superposition of the reflected incident waves givesrise to a standing wave pattern that is very well defined in space. Theprecise knowledge of the electromagnetic field configurations enables adielectric material of the rock/other material being treated to beplaced in the position of maximum electrical field strength, allowingmaximum heating ranges to be achieved. Single mode cavities are not asversatile as multi-mode cavities, but we have realised that by goingagainst traditional preferences for multi-mode cavities and using singlemode cavities, we can achieve much higher field strengths. Moreover, itis possible to tune a single mode cavity so as to present the maximumfield strength area in a position where it is wanted in the treatmentprocess plant.

However, single mode cavities/positioning material at maximum fieldstrength positions becomes unnecessary if multi-mode type cavities thatenable creation of sufficient power density are available, and they arenow. Thus we prefer multi-mode type cavities provided the power densitycreated within them is high enough.

Indeed, by having very high field strengths, we can heat materials thatare traditionally thought to be transparent to microwaves.

By having a power density that is much higher (e.g. 10¹⁵ Wm⁻³) thantraditionally achieved in multi-mode cavities, we achieve, very quickly,much higher thermal gradients across grain boundaries than previouslyachieved.

We have observed in trials 50%, and even 60% changes in strength withexposure times of less that 0.1 seconds. We have proved the principlethat it is not necessary to have tens of seconds of exposure tomicrowaves to get what is wanted.

FIG. 3A illustrates a single-mode microwave cavity 30. In this exampleit is suitable for processing minerals. Minerals, schematicallyillustrated at 32, enter a microwave pre-treatment zone 34 via an inputchannel 36. In the example shown in FIG. 3, the arrangement is vertical,and the mineral lumps/pieces 32 (which may typically be up to about 15cm in maximum dimension) fall under gravity through the input channel36, through the pre-treatment zone 34, and out beyond it through an exitchannel 38. The arrangement can be vertical, or inclined to the vertical(for slower feed rate of minerals), or even horizontal.

A microwave emitter 40 is provided in a microwave chamber 42, with theflow of minerals 32 passing through the microwave chamber 42, passingthrough the pre-treatment zone 34.

A reflector, or microwave short-circuit tuner, 44 is provided disposedopposite to the microwave emitter 40. Another reflector 46 is providedat the microwave emitter 40 (this reflector 46 may be optional).Microwave reflecting surfaces 48 also line the chamber 42.

Microwave emitter 40 emits microwaves, schematically illustrated as 49a; typically of 2.45 GHz, or 915 MHz (typically available microwavemagnetron frequencies). It may emit them continuously, or in pulsedmode. The microwaves are reflected back from reflector 44 and thereflected waves, schematically illustrated as 49 b interfere with theforward waves emitted by the emitter 40 and set up a standing wavepattern. This standing wave pattern has at least one maxima 52 (areawhere the power density is at a maximum) and minima (areas where thepower density is at a minimum).

Because maximum electric field strength is desired, so as to achieve thefastest rate of heating of different materials and hence the fastestdifferential heating, we ensure that the maxima 52 is at the place wherethe minerals 32 pass through the pre-treatment zone 34. Alternatively,put another way, we ensure that the materials 32 pass through thetreatment zone 34 at a place where the field strength is highest/highenough. We can control either, or both, of where the maxima occur, andwhere the material is disposed in the cavity. There may be only onemaximum in the standing wave.

We have a microwave generating device, and apply microwave energythrough a waveguide to a cavity, and couple and tune the cavity to themicrowave generating device (magnetron) to maximise electric fieldstrength in the area where the material to be treated is to be found inthe cavity.

FIG. 3B shows how the electric field strength experienced in the cavityvaries across the region of the cavity that is registered with theentrance channel 36. As will be seen, the electric field strength ishigher towards the middle cavity/aligned with the middle channel 36,than at the edges. This is due to constructive interference in thestanding wave that has been set up.

FIG. 4 a shows an embodiment similar to FIG. 3, but where the inputchannel 36′ directs materials being input into the treatment zone 34′specifically to a place where the standing wave of microwaves has amaxima 52′. In the example of FIG. 4 a, the mechanism for directing theflowing material through the position of maximum field of strength is afunnel-shaped channel which has an outlet adjacent the maxima 52′.Existing microwave machines can produce only one standing wave, with asingle maxima. This may or may not be true in the future.

FIG. 4 a also shows, conceptually, the ability to tune the standing wavein the cavity/treatment zone 34′ to control the position of the maxima.This is schematically shown by having reflector plate 44′ be movablerelative to the source of the microwaves 40′. The movable nature isshown by dotted alternative positions for the reflector 44′, and arrow56, which illustrates movement of the reflector.

FIG. 4 b is also relatively fanciful at present (since it is not knownhow to produce a standing wave as shown) but it schematicallyillustrates an alternative arrangement were the input channel 36″ has anumber of guide formations 58, which divide flowable material flowingthrough the treatment zone into different streams, referenced 60, eachof which encounters a different maxima 52″ of the standing wave set upin the microwave cavity. It will be appreciated that it is possible todo this by having funnels whose outlets correspond with maxima of thestanding wave. If it were possible to have a plurality of maxima then wecould do as suggested. That may be available in the future.

The power of the microwave emitter is between 1 and 100 kW, in thisexample it is 15 kW. The power density of the microwave emitter isbetween 10⁹ watts per cubic metre and 10¹⁵ or 10¹⁶ watts per cubicmetre. It may be possible to go higher that 10⁹ watts per cubic metre inpower density, but there is a potential for higher power densities tocause electric field breakdown of air within the material, which may bedetrimental (or which may not be detrimental).

We may prefer to have the size of the “lumps” passing through thetreatment chamber to be not too large (for example less than 20 cm orless than 15 cm in largest dimension).

FIG. 20A illustrates schematically an alternative to FIGS. 3A, 4A and 4Bfor a method of moving minerals 200 through a region for microwavetreatment. Minerals 200 are placed on a conveyer belt 206 whichcontinuously feeds the minerals 200 underneath a horn 204 and throughthe zone in which microwaves are present, denoted by dotted lines 212.The speed of the conveyer belt is set so as each piece of mineral has anexposure time (residence time in the microwave zone under the horn 204)of 1 ms and the process has a throughput of 1000 tonnes of mineral perhour. The microwave emitter produces four 1 μs pulses of radiation at afrequency of either 433 MHz, 915 MHz or 2.45 GHz every 1 ms, meaningthat each piece of mineral is subjected to four 1 μs pulses of microwaveradiation. An electric field strength approaching 30 kVcm⁻¹, which isthe field strength at which air breaks down, is created between thedotted lines 212. We need, in many embodiments to be below the electricfield strength at which air breaks down.

In other examples 10 pulses, or 50, or 100, or more pulses may beexperienced by the ore in the time it takes to traverse the microwavezone.

FIG. 20B illustrates schematically an alternative method of transferringminerals 200 through an area of microwave radiation denoted by dottedlines 212. A pneumatic pump is used to propel the minerals 200 throughthe area of microwave radiation 202 at a speed of up to 12 ms⁻¹. Thespeed of flow may be controllable. This enables a shorter exposure timeto the microwave radiation 202 than is possible with a conveyer belt anda higher throughput is achievable. In this example five 0.5 μs pulses ofmicrowave radiation of frequency 915/896 MHz are used to create therequired power density of the order of 10¹⁵ Wm⁻³. This raises thetemperature of the mineral as a whole by approximately 15° C., althougha temperature gradient of the order of tens, or several tens of ° C., or100-150° C. or so is created across the grain boundaries, which enablesthe mineral to be extracted in a downstream process with less energythan before.

FIG. 20C illustrates schematically another alternative method of passinga mineral, in this example coal 201, through an area of microwaveradiation denoted by the dotted lines 212. The coal 201 is continuouslyplaced at the top of a slide 210 and is moved through the area ofmicrowave radiation under gravity. The exposure time can be varied byaltering the gradient and length of the slide 210. In this example asingle 1 ms pulse of microwave radiation of frequency 433 MHz is used todehydrate the coal. In this example the coal is dried, and thepost-microwave process comprises burning the coal.

FIG. 2A shows a comminution plant 100 having an ore sizing mechanism 102which is adapted to ensure that ore leaving the sizing mechanism is of apredetermined maximum size, or range of sizes; a microwavepre-treatment/weakening unit 104 which comprises a unit such as that ofFIG. 3 or FIG. 4A or FIG. 4B or FIGS. 20A, 20B or 20C; a rod mill 106, afirst ball mill 108, a first hydrocyclone 110, a second ball mill 112,and a second hydrocyclone 114.

It will be appreciated that items 106 to 114 are prior art, and that thekey differentiation from the prior art is the microwave treatment unit104. However, it will be noted that microwave treatment unit 104 is aweakening unit, and that mechanical comminution is still performed afterweakening the ore. It will be noted that it may be necessary, or perhapsnot necessary, to mechanically condition/size the ore before it ismicrowaved in the unit 104.

It is desired in some examples to achieve a temperature gradient ofbetween 100 and 1500° C. across the grain boundary of a material of thefirst phase and the material of the second phase, so as to try to induceweaknesses/cracks at the grain boundary. In other examples we canachieve the fracturing/weakening we seek with lower temperaturegradients, for example perhaps 15-20° C., provided we induce thesegradients fast enough. The speed at which the temperature gradient isset up can enable us to use lower temperature gradients than previouslythought possible. A temperature gradient of a few tens of ° C. may beenough if very short (e.g. of the order of microsecond) microwave pulsesare used.

We realise that the change in strength of the material is a function ofpower density, that the temperature gradient is a function of powerdensity, that the shear strain is a function of temperature profile,that the shear stress is a function of the shear strain, and thatfailure will occur when the shear strain in the material exceeds theshear strength of the material. Thus, failure/weakening of the materialis intimately associated with power density (obviously assuming that thematerial contains a mixture of different materials with differentdielectric properties). One of the materials must be responsive tomicrowaves.

It is also a very strong advantage of the present invention that in manyembodiments it is a continuous process rather than a batch process. Byhaving a continuous flow of material through a treatment zone, we makethe process far more amenable to industrial application. The material tobe treated in many embodiments of the invention (whether that be toweaken the bond between two phases or for some other treatment purposes)passes through the cavity and experiences, short duration, microwavepulses that create high power densities. This is in contrast to batchprocesses where the material is loaded into a cavity with the microwavepower “off”, and then microwaves are applied, and then the microwavesare turned off, and then the material removed from the cavity.

Thus a microwave treatment zone can be established and a materialflowed/moved through it. In principle if the electric field strength ofthe microwaves vary across the treatment zone streams of material(possibly different material) may be arranged to pass through differentparts of the cavity so as to expose the different streams to differentelectric field strength microwaves. In order to get the most benefit outof any particular microwave generator (e.g. magnetron) one of thestreams will go through the maximum field strength region. In systemswhere there is no substantial variation in field strength across thecavity, or where the field strength is high enough at all places in thecavity, this point is moot.

The process may be semi-continuous (i.e. continuous flow of materialthrough the treatment for periods, and no flow for periods).

A further significant factor is the fact that we have realised that withsufficiently high field strengths to achieve sufficiently hightemperature gradients, the material does not have to be exposed tomicrowaves for very long. Traditionally, the prior art has exposedmaterials to microwaves for ten seconds or more, sometimes up to manyminutes. We believe that it is necessary to expose the material tomicrowaves, of sufficiently high field strength, for a second or less,and most preferably for less than about half a second, and even morepreferably for a time of the order of 0.2 seconds, or perhaps even less.FIG. 15 illustrates that 0.2 seconds is an appropriate time when most ofthe weakening to the material has been achieved. Similarly, FIG. 14shows that the difference in stress achieved between heating times of0.5 seconds and 0.25 seconds is not very great, especially in comparisonto the difference between 0.05 seconds and 0.25 seconds. This againpoints to about one quarter of a second being a suitable time to applyhigh-power microwaves for maximum result per unit cost.

However, for short duration pulsed microwaves (e.g. of the order of 1 μsfor a pulse) we have found that even shorter exposure to pulses iseffective. For example exposure to pulses for an aggregate time of theorder of 1 ms “hits” an ore with pulses of microwave, with substantialweakening of material.

Making the pre-treatment of two phase material with microwaves aneconomic proposition is improved by heating the materials withmicrowaves for a shorter time (much shorter) than the prior art suggestsis to be done.

The short exposure time to microwaves can be achieved in the examples ofequipment given by flowing the material through the treatment zone at ahigh rate (i.e. so that it flows through the high intensity maximaregions in about a quarter of a second or perhaps less). It might flowthrough in something of the order of a second or less in other examples.This has the double benefit of achieving the most heating effect perunit cost in microwave power, and also increasing throughput of materialthrough the heating zone—i.e. treating more material per second than waspreviously thought possible. This double benefit is very interesting.This also makes microwave pre-treatment even more financially feasible.

The invention is applicable to extracting one phase of material fromanother phase. For example it can be used to extract a liquid from asolid phase (e.g. extract water from a mineral, e.g. coal or talc).

In one example, we use 15 kW microwave applied for about 0.1 seconds.This gives an idea of what is meant by “high electric field”, or “highpower density”.

It is estimated that the comminution process to recover minerals fromores simply using mechanical treatment of the ores, without microwavetreatment, uses about 25 kW hours per ton of ore. It is estimated thatusing the present invention, this energy consumption could be reduced byhalf, or possibly even down to 80 or 90% less energy.

Since 60% to 70% of mineral processing plant costs relate to plantenergy consumption, this is a very significant reduction in the cost ofproducing minerals. Furthermore, by weakening the material to be brokenup by the comminution plant, there is less wear on the plant, theprocess is speeded up, and there is a higher throughput through themechanical comminution process. Moreover, because the materials areinter-granually broken, it is easier to recover the desired mineral. Theratio of recovery has been determined to be 3 or 4% better than if nomicrowave pre-treatment is used.

This experimental result of an increase of a few percent in recoveryrate is the first time that this has been observed. We subscribe theachievement of this effect to the higher electric field strengthmicrowaves that are applied.

We may have a resonance time/time for materials to be in the high fieldstrength region of the cavity of the order of 0.1 to 0.01 or even 0.001,seconds, or thereabouts. This is a very high throughput compared to theprior art.

Although gravity-fed systems are what are described in relation to FIGS.3, 4 a and 4 b, it is of course envisaged to have other feed mechanisms,such as pressure fed, conveyor belt fed, fluidised particle fed,centrifugal fed, or hopper fed, etc.

The moisture content of the ore may influence the selected powerdensity.

There may be a control processor controlling the tuning of the microwavecavity, and (in some embodiments) controlling the position of themaxima, or the position of the material in the cavity and controlling,optionally, the relative position of the flow of materials through thecavity and the position of the maxima. There may be a material-sensorproviding feedback signals to the control processor, and/or there may bean electric field probe to assist in monitoring the process, againproviding feedback signals to the control processor. Software for someembodiments to ensure that the physical position of the materials islined up with the physical position of the maximum intensity ofmicrowaves is also envisaged.

There may be flow-rate control means, optionally controlled by theprocessor, capable of varying the volume flow rate of material throughthe microwave cavity. This may be necessary to ensure that the materialexperiences the correct microwave conditions.

Particle size may influence the desired volume flow rate and/or powerdensity. There may be a particle size sensor, or a particle size inputmechanism (e.g. keyboard), for providing information to the controlprocessor relating to the particle size of the materials beingmicrowaved. The control processor may use this information to vary thelinear or volume flow-rate and/or power density.

There may be a controlled atmosphere in the cavity, for example anitrogen atmosphere or other inert gas atmosphere.

Other uses for the invention include separating two materials in ageneral sense—for example de-husking nuts (or making it easier toseparate two materials).

Moreover, the idea of achieving rapid heating using a very high fieldstrength very quickly applies to things that do not necessarily involveseparating materials. For example, drying materials, processing them tocause changes in the nature of the material, food processing.

The concept of creating a standing wave in a microwave cavity andestablishing where in the microwave cavity is the maximum electric fieldstrength of the standing wave and ensuring that material to be processedis disposed in the cavity at the position of maximum field strength, canbe applied to all sorts of physical processing. For example, rapidheating can cause fluffing of a material, and rapid heating can beuseful in chemical processing.

High power density for a short time, is a distinction over the art.

It will be appreciated that the conceptual, schematic, illustrative,waveforms of amplitudes of standing waves shown in the Figures are notbinding and are not restrictive. A three dimensional cavity may have amore complex standing wave, typically with only a single maxima whereconstructive interference creates a maximum/maximised field strengthregion, and the material to be processed will be disposed there.

The presence of the material in the cavity may possibly in somecircumstances influence where the maxima is found, and so the cavity mayneed to be tuned for use with a specific material of a specificvolume/shape, or flow rate, at a specific expected place in the cavity.Since electric field strength varies with a general square relationshipwith power density, electric field strength can fall off quite rapidlywith distance as one moves away from a position of maximumintensity—relatively careful alignment of the position of the materialto be processed and the cavity/standing wave may be desirable.

By “microwave” in the claims we mean at a first level microwaves atpermitted industrial microwave frequencies (currently 2.45 GHz, 915/896MHz and 433 MHz), and also microwaves generally (any frequency can beused if a Faraday cage is used to prevent electromagnetic pollution),and also RF heating frequencies, typically 27.12 MHz. We also intend tocover any electromagnetic radiation which heats two materialsdifferentially, i.e. infra red or ultra violet. “Microwave” in theclaims can be read as “electromagnetic radiation” (suitable for heatingthe materials concerned).

It will be appreciated that while the material is present in themicrowave treatment zone, it is not necessarily constantly exposed tomicrowave radiation. The material could have an exposure time tomicrowave radiation of the order of 5 μs, a few μs, tens of μs, a fewtens of μs, or a few, or tens of hundreds of μs which could be one pulseor a series of shorter pulses, which can be significantly less than theresidence time in the microwave treatment zone, which could be of theorder of seconds or tenths of a second.

It will also be appreciated that a plurality of cavities could be usedin series or parallel to achieve the desired throughput of multi-phasematerial, typically 1000 tonnes per hour. However, most embodiments willhave one cavity which is capable of processing 1000 tonnes ofmulti-phase material per hour.

It will further be appreciated that the temperature gradient created atthe boundaries of the separate phases within the multi-phase materialwill be ten, a few ten or several tens of ° C. but will be created overa very short time in order to create enough thermal stress to break thebonds between the different phases.

A large diamond mine can process 5 million tonnes of multi-phasematerial in a year as only approximately one part per million of themulti-phase material is diamond. Whereas a copper mine, where the copperis significantly more abundant than the diamond, can process ¼ milliontonnes per day.

The microwave cavity used can be of the order of 25 cm wide and 40 cmlong. Where a conveyor belt is used to deliver the mineral through themicrowave cavity, a typical belt velocity could be of the order of 4ms⁻¹ (perhaps 4 or 5 ms⁻¹). This would enable a residence time withinthe cavity of 0.1 seconds, however, the total microware treatment timemay be several micro second pulses within a millisecond, or onemicrosecond microwave pulse may produce a suitably high enough powerdensity.

We may apply 10-100 MW of microwave energy, but over a very short time(e.g. of the order of a small fraction of a second (e.g. a microsecondor so, or a millisecond or so).

There may be a total temperature rise of the bulk material of not muchmore than about 50° C. TABLE 9 References Bearman, R. A. Briggs, C. A.and Kojovic, T., 1997. The application of rock mechanics parameters tothe prediction of comminution behaviour. Minerals Engineering 10, 3255-264. Bearman R. A., 1999. The use of the point load test for therapid estimation of Mode I fracture toughness. International Journal ofRock mechanics and Miming Sciences., 257-263. Bieniawski Z. T., 1975.The Point Load Test in Engineering Practice, Engineering Geology. 9,1-11. Broch E. and Franklin J. A. (1972) The Point Load Strength Test.International Journal of Rock Mechanics and Mining Sciences., Vol. 9,669 to 697. Chen TT, Dutrizac JE, Haque KE, Wyslouzil W Kashyap S.,1984. The Relative Transparency of Minerals to Microwave Radiation. Can.Metall. Quart. 23, 1, 349-351. Harrison P. C. 1997. A fundamental studyof the heating effect of 2.45 GHz microwave radiation on minerals. Ph.D.Thesis, University of Birmingham. Itasca, 1995. Fast LangrangrianAnalysis of Continua, Version 3.3, Itasca Consulting Group Inc.,Minneapolis, Minnesota, USA Kingman, S. W. The Effect of MicrowaveRadiation on the comminution and beneficiation of minerals and ores.Ph.D. Thesis, University of Birmingham. Kingman SW Vorster W Rowson NA2000. The Influence of Mineralogy on Microwave Assisted Grinding.Minerals Engineering., 13, 3, 313-327. Napier-Munn TJ, Morell, S.,Morrison, R. D., Kojovic, T. 1996. Mineral Comminution Circuits. TheirOperation and Optimisation. JKMRC Monograph Series in Mining and MineralProcessing 2. JKMRC, Queensland, Australia. Rhodes M. 1998. Introductionto particle Technology, John Wiley and Sons Ltd, Chichester UK. Salsman,J. B. Williamson, R. L. Tolley W. K. and. Rice, D. A 1996. Short pulsemicrowave treatment of disseminated sulphide ores. MineralsEngineering., 9, 1, 43-54. Veasey TJ and Fitzgibbon KE. 1990. ThermallyAssisted Liberation of Minerals - A Review. Minerals Engineering,. 3, ½,181-185. Walkiwicz JW, Kazonich G, McGill SL. 1988. Microwave HeatingCharacteristics of Selected Minerals and Compounds., Minerals andMetallurgical Processing 5, 1, 39-42.

TABLE 1 Specific Heat Capacity as a Function of Temperature Specificheat capacity (J/Kg ° K) Mineral 298° K 500° K 1000° K Calcite 819 10511238 Pyrite 517 600 684

TABLE 2 Thermal Conductivity as a Function of Temperature Thermalconductivity (W/m ° K) Mineral 273° K 373° K 500° K Calcite 4.02 3.012.55 Pyrite 37.90 20.50 17.00

TABLE 3 Thermal Expansion Coefficient as a Function of TemperatureThermal expansion coefficient (1/° K) Mineral 373° K 473° K 673° K 873°K Calcite 13.1 × 10⁻⁶ 15.8 × 10⁻⁶ 20.1 × 10⁻⁶ 24.0 × 10⁻⁶ Pyrite 27.3 ×10⁻⁶ 29.3 × 10⁻⁶ 33.9 × 10⁻⁶ —

TABLE 4 Mechanical Properties of the Minerals Young's Residual Strengthdensity Modulus Poisson's Peak Strength (after 1% strain) Mineral Kg/m³Gpa Ratio φ° cMPa TMPa φ_(r)° c_(r)MPa T_(r)Mpa Pyrite 5018 292 0.16 5425 15 54 0.1 0 Calcite 2680 797 0.32 54 25 15 54 0.1 0

TABLE 5 Modelled Temperatures and Unconfined Compressive Strengths forVarious Microwave Heating Times (2.6 kW 2.45 Ghz, Microwave Cavity powerdensity between 3 × 10⁹ W/m³ and 9 × 10⁹ W/m³ Unconfined Heating timeMaximum Minimum compressive (seconds) temperature (° K) temperature (°K) strength (MPa) 0 300 300 126 1 350 300 126 5 460 320 123 15 700 40097 30 900 600 79

TABLE 6 Modelled Temperatures and Unconfined Compressive Strengths forVarious Microwave Heating Times (Microwave Cavity with a Power Densityof 1 × 10¹¹ watt/m³). Heating Unconfined time Maximum Minimumcompressive (seconds) temperature (° K) temperature (° K) strength (MPa)0 300 300 126 0.05 1200 300 57 0.25 1700 300 29 0.5 1900 300 26 1 1900300 25

TABLE 7 Breakage Parameters for 2.6 kW Multimode Cavity MicrowaveTreatment (power density between 3 × 10⁹ W/m³ and 9 × 10⁹ W/m³)time(secs) Is(50) Kic b A · b A 0 5.25 1.097 1.91 107.61 56.03 10 4.450.93 2.54 145.16 57.14 30 3.4 0.7106 4.22 238.56 56.63

TABLE 8 Breakage Parameters for 15 kW, 2.45 GHz (Power density 1 × 10¹¹W/m³ Single Mode Microwave Cavity Treated Ore time Is(50) Kic b A · b A0 5.25 1.097 1.91 107.01 56.03 0.1 1.8 0.376 11.83 772.67 65.31 0.2 1.250.2615 21.96 1513.41 68.91

1-26. (canceled)
 27. A method of increasing the yield of a mineralextracted from an ore having a plurality of phases of materialscomprising causing weakening of inter-phase boundaries by exposing saidore to high field strength microwaves for a time of less than 0.1second, the microwaves having a high enough field strength and beingapplied for a short enough time to cause differential thermal expansionbetween materials of different phases to cause weakening between phaseswhilst avoiding causing significant chemical changes to the ore, or atleast to the mineral to be extracted.
 28. A method according to claim 27wherein said ore is exposed to high field strength microwaves for a timeof less than 0.01 second.
 29. A method of microwave pre-treatment of amulti-phase material prior to a subsequent operation on the material toextract one material from the others, the method comprising providing acontinuous feed of the multi-phase material through a region in whichapplied microwave radiation is present, at a speed to allow a throughputof multi-phase material of at least 500 tonnes per hour, said microwaveradiation creating a power density of at least 10¹⁵ Wm⁻³, said materialexperiencing said microwave radiation for a time of the order of 1 ms orless, during which time it experiences one or a plurality of pulses ofenergy, and wherein the overall bulk temperature of the multi-phasematerial does not rise by more than 40° C., and wherein thermal stressis created between phase boundaries which is large enough to cause interphase fracturing, and wherein the temperature of said phases of saidmulti-phase material is kept low enough to avoid significant changes tothe chemical properties of said different phase materials.
 30. A methodaccording to claim 29 wherein the or each said pulse has a duration ofthe order of not more than microseconds.
 31. Apparatus for continuousprocessing of ore or rocks comprising means for applying high powerdensity microwaves, or high electric field strength microwaves, on acontinuous basis to ore or rocks passing through a microwave cavity orzone to weaken said ore or rocks, and feed means adapted to passsubsequently said continuous flow of said ore or rocks to a mechanicaltreatment machine adapted mechanically to break up said ore or rocks.32. A method of continuous processing of ore or rocks comprisingapplying at least one of (i) high power density microwaves, or (ii)high/electric field strength microwaves, on a continuous basis to ore orrocks passing through a microwave cavity or zone to weaken said ore orrocks at a speed that is fast enough to avoid causing substantialchemical change to said ore or rocks, and subsequently passing saidcontinuous flow of ore or rocks to a mechanical treatment machine andmechanically breaking up said ore or rocks.
 33. A method of microwavepre treatment of a multi-phase material prior to a subsequent operationon the material, said material having a first phase of material and asecond phase of material, the method comprising heating said materialwith microwaves, producing a power density of at least 10⁹ Wm⁻³ in acontinuous process in which said material moves into and through amicrowave treatment area and experiences exposure to said microwaves insaid treatment area for a time of the order of ½ second or less, saidtime being a short enough time to avoid causing substantial chemicalchanges to one, or both of said phases of said multi-phase material, andpassing said material out of said treatment area for said subsequentoperation.
 34. A method according to claim 33 wherein said materialexperiences microwaves in said treatment area for a time selected fromthe group consisting of: (i) of the order of 0.1 second or less; (ii) ofthe order of 0.01 second or less; and (iii) of the order of 0.001 secondor less.
 35. A method according to claim 27 wherein pulses of microwavesare emitted substantially continuously and said pulses have a durationfrom the group consisting of (i) of the order of 1 μs or less; (ii) ofthe order of 100 μs or less; (iii) of the order of 100 μs or less; (iv)of the order of 1 ms or less; and (v) of the order of 10 ms or less; ofthe order of 100 ms or less.
 36. A method according to claim 35 whereinsaid substance, whilst in said treatment area, experiences a series ofpulses of energy, said series having a number of pulses selected fromthe group consisting of: (i) of the order of 100 pulses or more; (ii) ofthe order of 50 pulses or more; (iii) of the order of 10 pulses or more;(iv) of the order of 5 pulses or more; (v) of the order of 2 pulses ormore; and (vi) of the order of one pulse.
 37. A method according toclaim 36 wherein said power density produced by the microwaves in thetreatment area is selected from the group consisting of the order of (i)10¹⁵ Wm⁻³ or more; and (ii) 10¹⁶ Wm⁻³ or more.
 38. A method according toclaim 34 wherein the bulk temperature of said material is raised by atemperature selected from the group consisting of: (i) less than 200°C.; and (ii) less than 150° C.; whilst said material is in saidtreatment area.
 39. A method according to claim 38 wherein said bulktemperature of said material is raised by a temperature selected fromthe group consisting of: (i) of the order of, or less than: (i) 50° C.;(ii) of the order of, or less than 20° C.; and (iii) of the order of, orless than 10° C.
 40. A method according to claim 34 wherein saidmaterial flows through said treatment area at a rate of at least 100tonnes an hour.
 41. A method according to claim 40 wherein said materialflows through said treatment area at a rate of the order of 1000 tonnesan hour or more.
 42. A method according claim 34 wherein said firstphase comprises a desired mineral and said second phase a rock substratesurrounding said mineral, and wherein said microwave energysignificantly weakens the bond strength between said mineral and saidsurrounding substrate by causing local differential thermal expansion.43. A method according to claim 42 wherein said microwave energy isapplied to said material for a short enough time to avoid causingsubstantial chemical changes to (i) said mineral; and/or (ii) both saidmaterial and substrate, that would detrimentally influence theefficiency of subsequent separation of said mineral and substrate.
 44. Amethod according to claim 33 wherein said first phase comprises amineral and said second phase comprises water, and wherein saidpre-treatment comprises dehydration, said electromagnetic energy dryingsaid mineral.
 45. A method according to claim 44 wherein said microwavesalso cause directly or indirectly fracturing or weakening of saidmineral.
 46. A method according to claim 45 wherein said first phasecomprises a hydrated mineral selected from the group consisting of: (i)coal; (ii) other hydrated material.
 47. A method of separating a mineralfrom an ore comprising pre-treating the ore in accordance with claim 33and subsequently comminuting said ore.
 48. A method according to claim34 wherein the power density within the treatment area produced by saidmicrowaves is selected from the group consisting of the order of 10¹⁰Wm⁻³, or more; 10¹¹ Wm⁻³, or more; 10¹² Wm⁻³, or more; 10¹³ Wm³, ormore, 10¹⁴ Wm⁻³, or more; and 10¹⁵ Wm⁻³, or more.
 49. A method ofrecycling articles which have parts made of different materials in themcomprising pre-treating said articles in accordance with claim 33 andthen mechanically stressing said articles in order to break them up andfacilitate the extraction of parts of said articles.
 50. Apparatus formicrowave treatment of material comprising: a microwave treatment zone;a microwave emitter disposed at said treatment zone; a materialtransporter adapted to transport material through the treatment zone;the arrangement being such that:— the emitter is adapted to emitmicrowaves that create a power density of at least 10⁹ Wm⁻³; and thematerial transporter is adapted to transport said material through thetreatment zone fast enough so that said material experiences significantmicrowaves in said zone for a time of the order of ½ second or less,said time being short enough to avoid causing substantial chemicalchange to said material.
 51. Apparatus according to claim 50 whereinsaid emitter is adapted to create a power density of microwaves in saidtreatment zone of at least 10¹⁵ Wm⁻³.
 52. Apparatus according to claim51 adapted to cause said material to experience microwaves for a timeselected from the group consisting of (i) of the order of 0.1 second orless; (ii) of the order of 0.01 second or less; and (iii) of the orderof 0.001 second or less.
 53. Apparatus according to claim 52 adapted totransport of the order of 1000 tonnes of material an hour through saidtreatment zone.
 54. Apparatus according to claim 52 wherein said emitteris adapted to produce microwave pulses with a duration selected from thegroup consisting of (i) of the order of a microsecond; (ii) of the orderof tens of microseconds; and (iii) of the order of hundreds ofmicroseconds, or less.
 55. Apparatus according to claim 54 adapted toapply many pulses of microwaves to said material, whilst said materialis in said treatment zone.
 56. A method according to claim 32 whereinthe exposure of said ore or rocks to said high field strength microwavesis for a time selected from the group consisting of: (i) of the order ofhalf a second or less; (ii) of the order of a quarter of a second orless; (iii) of the order of 0.1 second or less; and (iv) of the order of0.01 seconds or less.